Thermal storage system

ABSTRACT

A thermal storage system includes a first tank and a second tank thermally interfaced with the first tank. A pump is connected between the first tank and the second tank to move a fluid from the first tank to the second tank. A first heat exchanger includes a heat-exchanging portion that is located within the first tank. A second heat exchanger includes another heat-exchanging portion that is located within the second tank.

BACKGROUND

This disclosure relates to thermal systems that utilize thermal storagefluids.

There are many different types of thermal systems that use a thermalstorage fluid for storing thermal energy. As an example, solar powerplants utilize a thermal storage fluid to capture solar energy for thepurpose of generating electricity. A solar power plant may include asolar collector system that directs solar energy toward a centralreceiver. The solar energy heats a thermal storage fluid, such as amolten salt or phase change material, which circulates though thereceiver. The heated thermal storage fluid may then be used to producesteam and drive a turbine to generate electricity. The thermal storagefluid may be stored or circulated through a series of tanks. Typically,some of the tanks store cool fluid and, when needed, provide the coolfluid to the receiver. Other tanks store heated fluid from the receiver,for producing the steam.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the disclosed examples willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example thermal storage system within a solarpower system.

FIG. 2 illustrates another example thermal storage system.

FIG. 3 illustrates another example thermal storage system that isconfigured for use with a phase change material.

FIG. 4 illustrates another example thermal storage system that isconfigured with sets of tanks as modules.

FIG. 5 illustrates another example thermal storage system havingmultiple small tanks within a larger tank.

FIG. 6 illustrates another example thermal storage system havingmultiple small tanks arranged around a perimeter of a larger tank.

FIG. 7 illustrates another example thermal storage system havingadjustable volume tanks.

FIG. 8 illustrates another example thermal storage system having agravity pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates selected portions of an example thermal storagesystem 20 that may be used to handle a thermal storage fluid, such as amolten salt, in a thermally efficient manner. Although selectedcomponents of the thermal storage system 20 are shown in this example,it is to be understood that additional components may be utilized withthe thermal storage system 20, depending on the particularimplementation and needs of a system, for example.

As illustrated, the thermal storage system 20 is arranged within a solarpower system 22, which will be described in more detail below.Alternatively, the thermal storage system 20 may be arranged withinother types of systems, such as nuclear systems, electric arc furnacesystems, or other thermal systems that utilize thermal storage fluidthat would benefit from the disclosed examples.

The thermal storage system 20 includes a first tank 24 and a second tank26 that is thermally interfaced with the first tank 24. For example, thetanks 24 and 26 share a common boundary or adjoining wall through whichheat exchange occurs. The wall may be modified mechanically orchemically to enhance heat transfer from solid deposition, for example.In this case, the first tank 24 and the second tank 26 are generallyhollow, and the first tank 24 is arranged at least substantially withinthe interior of the second tank 26. In the example, the first tank 24 iscompletely within the second tank 26. Alternatively, a portion of thefirst tank 24 may extend from the second tank 25.

Any “heat loss” (from a thermal storage fluid within the first tank 24)through the walls of the first tank 24 would be lost into the interiorof the second tank 26. Rather than being dissipated to the ambientsurroundings as might be the case with a single tank, the heat loss fromthe first tank 24 is absorbed by the thermal storage fluid in the secondtank 26. Thus, the arrangement of the thermal storage system 20facilitates achievement of enhanced thermal efficiency. Additionally,one or both of the tanks 24 and 26 may include high heat capacitymembers, such as capsules containing compressed gas or capsules made ofceramic material, to enhance the heat storing capacity of the thermalstorage system 20.

In the illustrated example, each of the first tank 24 and the secondtank 26 is generally cylindrical in shape. The cylindrical shapeprovides a relatively low surface area per volume to facilitate avoidingheat loss from the tanks 24 and 26. Alternatively, it is to beunderstood that other shapes may be selected for the tanks 24 and 26,such as but not limited to, square shapes or other geometric shapes.

The first tank 24 includes a floor 24 a, sidewalls 24 b, and a top 24 c.Likewise, the second tank 26 includes a floor 26 a, sidewalls 26 b, anda top 26 c. In some examples, the tops 24 c and 26 c may be separate anddistinct pieces that enclose the interior volumes of the tanks 24 and26. However, alternatively the tops 24 c and 26 c may actually be asingle, common top that encloses the interior volumes of both tanks 24and 26. That is, the top surfaces of the tanks 24 and 26 may be flush.Similarly, the floors 24 a and 26 a may be separate and distinct, orcommon.

In the thermal storage system 20, a pump 28 is connected between thefirst tank 24 and the second tank 26 to move the thermal storage fluidfrom the first tank 24 to the second tank 26. As shown, the pump 28 isexterior to the tanks 24 and 26. However, the pump 28 may alternativelybe located within the interior of the second tank 26 or the first tank24.

The thermal storage system 20 further includes a first heat exchanger 30having a heat exchanging portion 30 a within the first tank 24. A secondheat exchanger 32 likewise includes a heat exchanging portion 32 a thatis within the second tank 26. As shown, the heat exchangers 30 and 32are coil-type heat exchangers that are capable of circulatingheat-exchanging fluids, such as water, carbon dioxide, combinationsthereof, or other suitable fluids. Alternatively, other types ofheat-exchangers may be used. In this case, the heat exchangers 30 and 32are connected such that the heat-exchanging fluid flowing through theheat exchangers 30 and 32 flows through the second heat exchanger 32 inthe second tank 26 and then to the first heat exchanger 30 in the firsttank 24. Alternatively, the heat exchangers 30 and 32 are not connectedsuch that heat-exchanging fluids flow independently through each.

In some examples, the first tank 24 may be mounted above the floor 26 aof the second tank 26 such that there is a space 34 that spans betweenthe floor 24 a of the first tank 24 and the floor 26 a of the secondtank 26. The first tank 24 may include legs or other suitable hardwarefor mounting above the floor 26 a.

Similarly, the top 24 c of the first tank 24 may be below, or spacedfrom, the top 26 c of the second tank 26 such that there is a space 36between the tops 24 c and 26 c. A thermal storage of fluid may flowthrough the spaces 34 and 36, if the thermal storage system 20 isdesigned with such spaces 34 and 36. In some examples, providing one orboth of the spaces 34 and 36 for the flow of the thermal storage fluid,rather than having the top 24 c or bottom 24 a of the first tank 24exposed to the ambient surrounding environment, allows a greater degreeof heat loss from the first tank 24 to be received into the thermalstorage fluid in the second tank 26.

In the example illustrated, the second tank 26 may also experience heatloss to the surrounding ambient environment. Generally, the thermalstorage fluid held within the interior of the second tank 26 loses moreheat to the surrounding environment than is absorbed from the first tank24. Therefore, the thermal storage fluid within the second tank 26 isnormally cooler than the thermal storage fluid within the first tank 24.

The result of the difference in temperature between the thermal storagefluid in the second tank 26 and the thermal storage fluid in the firsttank 24 is that the heat lost from the first tank 24 to the second tank26 may be used to preheat the heat-exchanging fluid flowing through thesecond heat exchanger 32 before further heating the heat-exchangingfluid in the first tank 24. Thus, the heat is used more efficiently andthe thermal storage system 20 may be made more compact than conventionmolten salt systems that utilize separate cold and hot molten salttanks. The heat-exchanging fluid from the first heat exchanger 30 maythen be provided to a component 60, such as a turbine of a generator ora Brayton cycle.

The thermal storage system 20 may also include an external fluid circuit70 for moving the thermal storage fluid to and from the tanks 24 and 26.In this case, the external fluid circuit 70 forms a portion of the solarpower system 22. The solar power system 22 also includes a solarreceiver 72 connected within the external fluid circuit 70 and throughwhich the thermal storage fluid can be circulated. Pumps or othercontrol components may be incorporated, as is generally known. At leastone solar collector 74 is operative to direct solar energy 76 toward thesolar receiver 72 to heat the thermal storage fluid as it circulatesthrough the solar receiver 72.

The external fluid circuit 70 is connected at an inlet 80 of the firsttank 24 and at an outlet 82 of the second tank 26 to move the thermalstorage fluid from the outlet 82 to the inlet 80. In this example, thetanks 24 and 26 also include respective circulation pumps 86 a and 86 b,which facilitate moving the thermal storage fluid within the tanks 24and 26 to more uniformly distribute heat.

FIG. 2 illustrates another example thermal storage system 120 that issomewhat similar to the thermal storage system 20 illustrated in FIG. 1.In this disclosure, like reference numerals designate like elementswhere appropriate, and reference numerals with the addition ofone-hundred or multiples thereof designate modified elements that areunderstood to incorporate the same features and benefits of thecorresponding original elements. In this case, the thermal storagesystem 120 additionally includes a third tank 190. The first tank 124 isat least substantially within the interior of the second tank 126, andthe second tank 126 is at least substantially within the interior of thethird tank 190. Thus, heat loss from the second tank 126 will beabsorbed by the thermal storage fluid within the interior of the thirdtank 190. Thus, the arrangement of the thermal storage system 120facilitates achievement of enhanced thermal efficiency.

An additional pump 128 a is connected between the second tank 126 andthe third tank 190 to move the thermal storage fluid from the secondtank 126 to the third tank 190. Except for the pumps 128 and 128 a, theinteriors of the tanks 124, 126, and 190 are generally sealed from eachother. In this case, the thermal storage fluid enters the thermalstorage system 120 at the inlet 180 of the first tank 124. The thermalstorage fluid may then move into the second tank 126 via the pump 128and then into the third tank 190 via the pump 128 a. The thermal storagefluid may then be received into the external fluid circuit 170 from theoutlet 182 of the third tank 190.

The use of the third tank 190 provides the benefit of selectivelysuperheating the heat-exchanging fluid that circulates through the heatexchangers 130 and 132 by making more efficient use of the heat in thesystem, as described above. In this case, superheating may be used withpower cycles to generate electricity with improved efficiency.

FIG. 3 illustrates another example thermal storage system 220 that is amodification of the thermal storage system 20. In this example, thefirst tank 24 and the second tank 26 contain a phase change material asthe thermal storage fluid. The phase change material circulates betweenthe first tank 24 and the second tank 26. The phase change material isnot limited to any particular type and may be, for example, a salt, aeutectic alloy, or an organic material. In this case, the external fluidcircuit 270 connects to a heat exchanger 282 having a heat exchangingportion 282 a within the first tank 24 to transfer heat from the solarreceiver 72 to the phase change material.

The heat exchanger 230 and heat-exchanging portion 230 a transfer heatfrom the phase change material to a heat-exchanging fluid, such assteam, carbon dioxide, or a carbon dioxide/steam mixture, which maydrive component 60 (e.g., a turbine of a generator, Brayton cycle,etc.).

In another example thermal storage system 420 illustrated in FIG. 4, theset of first and second tanks 24 and 26 (or 124 and 126) may be providedas a compact module 390 and used side-by-side or in a stackedconfiguration with one or more other compact modules 390 and externalcircuit 370. Although four modules 390 are shown, fewer or additionalmodules 390 may alternatively be used. The modules 390 provide thebenefit of easier maintenance of an individual one of the modules 390without having to shut down the entire thermal storage system 420. Thatis, each of the modules 390 can be isolated from the system, whilemaintaining operation of the system, for maintenance, cleaning, or thelike. Additionally, use of relatively small modules compared to large,separate hot and cold tanks that are used in conventional systems,facilitates reduction of stagnant zones within the tanks 24 and 26,which improves the efficiency of the system.

FIGS. 5-9 illustrate additional example configurations for thermallyinterfacing the tanks 24 and 26. In the examples, the tanks 24 and 26share a common wall or boundary that allows the tanks 24 and 26 toexchange heat. For clarity, the other components in the system, such aspumps (e.g., mechanical or gravity) connected between the tanks and heatexchangers as described herein, are not shown but may be similar toother disclosed examples. In FIG. 5, the thermal storage system includesmultiple small tanks 24 that are stacked within the larger tank 26. Thesmall tanks 24 each touch at least two other neighboring tanks 24 suchthat the tanks 24 are thermally interfaced with each other in additionto the larger tank 26. In FIG. 6, multiple small tanks 24 are arrangedaround the perimeter of the larger tank 26. Each of the tanks 24 touchestwo other neighboring tanks 24 as well as the larger tank 26.

In FIG. 7, the tanks 24 and 26 each have adjustable walls such that thevolumes of the tanks 24 and 26 are adjustable. For instance, the volumesof the tanks 24 and 26 are adjusted in response to the heat-storingcapacity that is needed at a particular time. As an example, morestoring capacity is desired at night when energy consumption is low andless storing capacity is needed during the day when consumption is high.

In FIG. 8, the first tank 24 is arranged within the second tank 26 andis mounted near the top of the second tank 26 such that gravity servesas a pump to feed the thermal storage fluid from the first tank 24 tothe second tank 26.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

1. A thermal storage system comprising: a first tank; a second tankthermally interfaced with the first tank; a pump connected between thefirst tank and the second tank to move a fluid from the first tank tothe second tank; a first heat exchanger having a heat-exchanging portionwithin the first tank; and a second heat exchanger having aheat-exchanging portion within the second tank.
 2. The thermal storagesystem as recited in claim 1, wherein the first tank is at leastsubstantially within the second tank.
 3. The thermal storage system asrecited in claim 1, further comprising a third tank, wherein the firsttank is at least substantially within the second tank, and the secondtank is at least substantially within the third tank.
 4. The thermalstorage system as recited in claim 3, further comprising another pumpconnected between the second tank and the third tank to move the fluidfrom the second tank to the third tank.
 5. The thermal storage system asrecited in claim 1, wherein the first tank and the second tank are eachcylindrical.
 6. The thermal storage system as recited in claim 1,wherein the second tank includes at least a floor and sidewalls, and thefirst tank is mounted above the floor of the second tank and at leastsubstantially within the second tank.
 7. The thermal storage system asrecited in claim 1, further comprising an external fluid circuitconnected to an inlet of the first tank and an outlet of the second tankto circulate fluid received from the second tank into the first tank. 8.The thermal storage system as recited in claim 7, further comprising asolar receiver connected within the external fluid circuit and throughwhich the fluid can be circulated, and at least one solar collectoroperative to direct solar energy toward the solar receiver to heat thefluid flowing therethrough.
 9. The thermal storage system as recited inclaim 1, wherein each of the first tank and the second tank includes arespective circulation pump.
 10. The thermal storage system as recitedin claim 1, wherein the second heat exchanger is fluidly connected tothe first heat exchanger.
 11. The thermal storage system as recited inclaim 1, further comprising additional first tanks that are thermallyinterfaced with the second tank, wherein the first tanks are within thesecond tank.
 12. The thermal storage system as recited in claim 1,further comprising additional first tanks that are thermally interfacedwith the second tank, wherein the first tanks are arranged around theperimeter of the second tank.
 13. The thermal storage system as recitedin claim 1, wherein each of the first tank and the second tank haveadjustable volumes.
 14. A solar power system comprising: a solarreceiver through which a working fluid can be circulated; at least onesolar collector operative to direct solar energy toward the solarreceiver to heat the working fluid; and a thermal storage system fluidlyconnected with the solar receiver such that the working fluid can alsobe circulated through the thermal storage system, the thermal storagesystem including a first tank, a second tank thermally interfaced withthe first tank, a pump connected between the first tank and the secondtank to move a fluid from the second tank to the first tank, a firstheat exchanger having a heat-exchanging portion within the first tank,and a second heat exchanger having a heat-exchanging portion within thesecond tank.
 15. The solar power system as recited in claim 14, furthercomprising a turbine connected with the first heat exchanger.
 16. Thesolar power system as recited in claim 14, further comprising multipleadditional thermal storage systems, wherein all the thermal storagesystems are arranged side-by-side.
 17. A method for use with a thermalstorage system that includes a first tank, a second tank thermallyinterfaced with the first tank, a pump connected between the first tankand the second tank to move a thermal storage fluid from the first tankto the second tank, a first heat exchanger having a heat-exchangingportion within the first tank, and a second heat exchanger having aheat-exchanging portion within the second tank, the method comprising:heating the thermal storage fluid; moving the heated thermal storagefluid from the first tank to the second tank; heating a heat-exchangingfluid within the heat-exchanging portion of the second heat exchangerusing the heated thermal storage fluid within the second tank; furtherheating the heat-exchanging fluid within the heat-exchanging portion ofthe first heat exchanger using the heated thermal storage fluid withinthe first tank.
 18. The method as recited in claim 17, including drivinga turbine using the heat-exchanging fluid.
 19. The method as recited inclaim 17, wherein the heating of the thermal storage fluid includesheating using solar energy.
 20. The method as recited in claim 17,wherein the heated thermal storage fluid is a phase change material. 21.The method as recited in claim 17, wherein the heating of the thermalstorage fluid includes heating using a second heat-exchanging fluidreceived from a solar receiver.